Below is some supporting info and the formula for absolute mode uncertianty on a PM600 module sample calibration certificate:
Here is some info to better understand the RSS (Root Sum Square) formula given.
The 1 year specification in gauge mode has a coverage factor of 2 and the absolute mode stability has rectangular distribution, so it has a coverage factor of 1.73 and thus why seeing these two numbers as dividers in the formula. This 1.73 number is the square root of 3 if wondering its source.
To get a combined specification in absolute mode at a coverage factor of 2 or approximately 95%, they have to be normalized to a coverage factor of 1, combined using RSS, then multiplied by 2 to get back to k=2.